Bioregulators for Pest Control - American Chemical Society

4 Elicitation of Disease Resistance in Plants by the Expression of Latent Genetic Information Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 27, 2018 | https://pubs.acs.org Publication Date: April 26, 1985 | doi: 10.1021/bk-1985-0276.ch004

STEVEN D. SALT and JOSEPH KUĆ Department of Plant Pathology, University of Kentucky, Lexington, KY 40546-0091 Cucumber, melon, tobacco, and bean varieties, supposedly l a c k i n g s i g n i f i c a n t resistance to particular diseases, can be made highly resistant systemically by limited infection prior to severe exposure to pathogens. Injury and non-specific stress do not e l i c i t persistent systemic resistance. These findings suggest that plants generally possess genetic information for disease resistance mechanisms and that resistance is generally determined by the speed and magnitude of response to pathogens. We s h a l l present aspects of our investigations into immunization of tobacco, c u c u r b i t s , and beans (Phaseolus vulgaris L.) and b r i e f l y review other workers' i n v e s t i g a t i o n s into bioticallyand chemically-induced disease resistance of plants. Enhanced resistance to disease in plants after an i n i t i a l infection has fascinated observers for over 100 years. A review of the subject by Chester in 1933 contains 201 references (1). "Immunization", "acquired systemic resistance", or "induced resistance" of plants have been reviewed in recent years (2-11). We shall not exhaustively review the literature, but shall focus on general principles and phenomena of particular relevance to the use of "plant immunization" for the practical control of disease. This paper w i l l stress examples from our own research program, but will also include literature citations to provide the reader with an appreciation of important research c o n t r i b u t i o n s of others previously and presently active in the f i e l d . Most examples presented w i l l deal with fungal, bacterial, or v i r a l diseases of crop plants, but similar principles may apply to infestations by nematodes and, possibly, insects. Protection of Plants Against Pests Immunization of plants via priming for expression of latent genetic information encoding disease resistance mechanisms may be

0097-6156/85/0276-0047$06.50/0 © 1985 American Chemical Society

Hedin et al.; Bioregulators for Pest Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 27, 2018 | https://pubs.acs.org Publication Date: April 26, 1985 | doi: 10.1021/bk-1985-0276.ch004

i n t r o d u c e d by c o n t r a s t i n g i t w i t h o t h e r s t r a t e g i e s u t i l i z e d f o r p r o t e c t i n g p l a n t s from d i s e a s e s The s t r a t e g i e s c o n s i d e r e d can be c a t e g o r i z e d as: e c o l o g i c a l , p e s t i c i d e s , a n t i t o x i n s , a l t e r a t i o n s of p l a n t p h y s i o l o g y , n o n s p e c i f i c p h y t o a l e x i n i n d u c t i o n , and plant s e n s i t i z a t i o n (immunization). Ecological. This h i s t o r i c a l l y venerable strategy i s characterized by t a c t i c s d e s i g n e d to m i n i m i z e exposure o f p l a n t s to pathogens and make e n v i r o n m e n t a l c o n d i t i o n s u n f a v o r a b l e f o r d i s e a s e development. Measures i n c l u d e c r o p r o t a t i o n , s a n i t a t i o n , and q u a r a n t i n e . Such p r o c e d u r e s are v a l u a b l e and worthy of f u r t h e r d e v e l o p m e n t , but they are g e n e r a l l y of l i m i t e d e f f e c t i v e n e s s i n p r o v i d i n g r e l i a b l y h i g h y i e l d and q u a l i t y o f c r o p s i n a r e a s w i t h s e v e r e o r p e r s i s t e n t d i s e a s e problems. Pesticides. The a p p l i c a t i o n o f exogenous c h e m i c a l s t o x i c to p l a n t pathogens and p e s t s has been a major s t r a t e g y f o r p l a n t protection i n t e c h n o l o g i c a l l y d e v e l o p e d areas for s e v e r a l decades. This s t r a t e g y i s w e l l s u i t e d to mechanized, h i g h t e c h n o l o g y a g r i c u l t u r e , and has v e r y s u c c e s s f u l l y enhanced c r o p p r o d u c t i o n . Numerous and s e r i o u s l o n g - t e r m problems have, however, become e v i d e n t . Heavy r e l i a n c e on, and c o n f i d e n c e i n , p e s t i c i d e s has caused other strategies t o become r e l a t i v e l y neglected. The expense of p e s t i c i d e s and n e c e s s a r y a n c i l l a r y e q u i p m e n t , as w e l l as t e c h n o l o g i c a l e x p e r t i s e r e q u i r e d f o r t h e i r e f f e c t i v e and s a f e use, are burdensome and o f t e n beyond the r e a c h o f farmers i n d e v e l o p i n g nations. H e a v y and i m p r o p e r use o f p e s t i c i d e s has l e d to the a p p e a r a n c e o f r e s i s t a n t p e s t s and c o n s e q u e n t l o s s o f p e s t i c i d e efficacy. M o s t s e r i o u s l y , t h e d a n g e r o f i n j u r y to n o n - t a r g e t organisms, e s p e c i a l l y t o x i c i t y , t e r a t o g e n i c i t y , and carcinogenicity to humans, has l e d to cumbersome and e x p e n s i v e r e s t r i c t i o n s on the development, s a l e , and use o f p e s t i c i d e s . Antitoxins. This i s a s t r a t e g y of disarmament i n which a p p l i e d c h e m i c a l s are not d i r e c t l y t o x i c to pathogens, but i n t e r f e r e w i t h t h e i r mechanisms f o r p a t h o g e n e s i s , e.g., p e n e t r a t i o n , m a c e r a t i o n of t i s s u e s , or i n d u c t i o n of w i l t i n g or a b n o r m a l growth. Few s u c h compounds have been d e v e l o p e d f o r commercial use. Tricyclazoles a p p e a r t o f u n c t i o n by i n h i b i t i n g t h e m e l a n i z a t i o n o f fungal a p p r e s s o r i a and thus reduce p e n e t r a t i o n i n t o h o s t p l a n t s ( 12-14). Other r e p o r t e d examples i n c l u d e i n a c t i v a t i o n of p i r i c u l a r i n , t o x i n o f the r i c e b l a s t f u n g u s P i r i c u l a r i a o r y z a e , by f e r u l i c and c h l o r o g e n i c a c i d s (15), and i n h i b i t i o n o f Fusarium w i l t symptoms i n tomato by c a t e c h o l , which at e f f e c t i v e c o n c e n t r a t i o n s appears non t o x i c t o t h e f u n g u s and n e i t h e r p r e v e n t s nor r e d u c e s i n f e c t i o n (16). The b i o c h e m i s t r y of p a t h o g e n e s i s i n p l a n t d i s e a s e , however, i s i n an e a r l y s t a g e o f i n v e s t i g a t i o n and the r a t i o n a l d e s i g n o f t o x i n i n h i b i t o r s or o t h e r a n t i p a t h o g e n e t i c compounds may be e v e n more d i f f i c u l t t h a n t h a t o f p e s t i c i d e s . Random s c r e e n i n g o f compounds f o r a n t i t o x i n a c t i v i t y has i n h e r e n t d i f f i c u l t i e s f o r b i o a s s a y , and the a p p l i c a t i o n o f a n t i t o x i n s s u f f e r s f r o m t h e d i f f i c u l t i e s encountered i n the e n v i r o n m e n t a l r e l e a s e of c h e m i c a l s as c i t e d f o r p e s t i c i d e s .


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A l t e r a t i o n s o f P h y s i o l o g y . S u s c e p t i b l e p l a n t s p e c i e s or c u l t i v a r s may be r e n d e r e d resistant t o d i s e a s e by so a l t e r i n g their p h y s i o l o g y , e.g., h o r m o n a l b a l a n c e s , i o n f l u x e s , c o n s t i t u t i v e secondary m e t a b o l i s m , or r e s p i r a t o r y r a t e s , as to render them u n f i t o r h o s t i l e e n v i r o n m e n t s f o r p a t h o g e n d e v e l o p m e n t , i . e . , t o make them n o n - h o s t s . T h i s c o u l d be a c c o m p l i s h e d , f o r e x a m p l e , by the use o f growth r e g u l a t o r s or b r e e d i n g . T h i s approach may not p r o v e g e n e r a l l y p r a c t i c a b l e , however, because crop p l a n t s , o v e r the ages, have been s e l e c t e d p r e c i s e l y f o r t h o s e p h y s i o l o g i c a l c h a r a c t e r i s t i c s w h i c h make them more agronomica1 l y d e s i r a b l e than f r e q u e n t l y hardier w i l d r e l a t i v e s . D r a s t i c c h a n g e s i n c o n s t i t u t i v e p h y s i o l o g y may r e d u c e a g r o n o m i c f i t n e s s e v e n i f d i s e a s e r e s i s t a n c e i s enhanced. Breeders are o f t e n f a c e d w i t h the p r o b l e m o f r e i n c o r p o r a t i n g y i e l d and q u a l i t y f a c t o r s back i n t o d i s e a s e - a n d i n s e c t - r e s i s t a n t p l a n t s . N o n - s p e c i f i c or C o n s t i t u t i v e P h y t o a l e x i n I n d u c t i o n . Many p l a n t s a r e r e p o r t e d t o p r o d u c e a v a r i e t y o f low m o l e c u l a r weight, a n t i m i c r o b i a l compounds ( p h y t o a 1 e x i n s ) i n r e s p o n s e t o i n f e c t i o n . These compounds r a p i d l y a c c u m u l a t e to h i g h c o n c e n t r a t i o n s i n p l a n t t i s s u e s i m m e d i a t e l y a d j a c e n t to s i t e s of i n f e c t i o n i n r e s i s t a n t c u l t i v a r s , and they f u n c t i o n to r e s t r i c t d e v e l o p m e n t of f u n g a l and b a c t e r i a l pathogens o f p l a n t s (17,18). Susceptible c u l t i v a r s may u l t i m a t e l y a c c u m u l a t e as much o r more t o t a l p h y t o a l e x i n s d u r i n g d i s e a s e development, but a c c u m u l a t i o n i s s l o w e r and d i f f u s e , and l o c a l i z e d c o n c e n t r a t i o n s i n advance of the pathogen appear to be i n s u f f i c i e n t to i n h i b i t pathogen development. Many a b i o t i c agents, i n c l u d i n g heavy m e t a l s a l t s (19), heat (20), c h l o r o f o r m v a p o r s ( 2 1 ) , u l t r a v i o l e t i r r a d i a t i o n ( 2 2 ) , f u n g i c i d e s s u c h as maneb and b e n o m y l ( 2 3 ) , and e v e n i n n o c u o u s s u b s t a n c e s s u c h as s u c r o s e (24) e l i c i t p h y t o a l e x i n a c c u m u l a t i o n i n some p l a n t s . P r o p o s a l s have b e e n made t h a t g e n e r a l n o n - s p e c i f i c i n d u c t i o n o f p h y t o a l e x i n a c c u m u l a t i o n i n p l a n t s may be an effective and " n a t u r a l " method o f p l a n t p r o t e c t i o n from p e s t s . U n f o r t u n a t e l y , p h y t o a l e x i n s , and the e f f e c t i v e c o n c e n t r a t i o n s o f p h y t o a l e x i n e l i c i t o r s , a r e o f t e n t o x i c a n t s which i n j u r e or k i l l p l a n t c e l l s (25). In d i s e a s e r e s i s t a n c e i n v o l v i n g p h y t o a l e x i n a c c u m u l a t i o n , the p l a n t s a c r i f i c e s a few o f i t s own c e l l s i m m e d i a t e l y a d j a c e n t to i n v a d i n g pathogens i n o r d e r t o s a v e the e n t i r e p l a n t . General or c o n s t i t u t i v e i n d u c t i o n o f p h y t o a l e x i n a c c u m u l a t i o n may p r o t e c t p l a n t s a g a i n s t i n f e c t i o u s d i s e a s e , but i t would l i k e l y be d i s a s t r o u s l y c o u n t e r p r o d u c t i v e to o v e r a l l p l a n t h e a l t h because o f a u t o t o x i c i t y and t h e need f o r c o n t i n u o u s d i v e r s i o n o f m e t a b o l i c e n e r g y and primary m e t a b o l i t e s into secondary metabolism. In a d d i t i o n , phytoalexins require higher c o n c e n t r a t i o n s i n p l a n t t i s s u e s than commercial synthetic p e s t i c i d e s to achieve comparable effectiveness. They a l s o are not t r a n s l o c a t e d w i t h i n p l a n t s , and a r e r a p i d l y d e g r a d e d by p l a n t s and many p a t h o g e n s . To m a i n t a i n e f f e c t i v e l y h i g h s y s t e m i c l e v e l s o f p h y t o a l e x i n s , t h e r e f o r e , would l i k e l y r e q u i r e repeated a p p l i c a t i o n s of p h y t o t o x i c elicitor s u b s t a n c e s or the b r e e d i n g o f p l a n t s w i t h c o n s t i t u t i v e l y h i g h l e v e l s of p h y t o a l e x i n s . F u r t h e r m o r e , many p h y t o a l e x i n s may be t o x i c t o mammals, and p l a n t t i s s u e c o n t a i n i n g h i g h total c o n c e n t r a t i o n s may be u n p a l a t a b l e o r h a z a r d o u s f o r c o n s u m p t i o n (25).



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S e n s i t i z a t i o n of Plants. This strategy f o rplant p r o t e c t i o n i s c o n c e p t u a l l y , i f not m e c h a n i s t i c a l l y , a n a l o g o u s t o immunization v i a v a c c i n a t i o n o f mammals. An i n i t i a l s t i m u l u s e l i c i t s a l o n g - t e r m s u b t l e a l t e r a t i o n i n a p l a n t such that subsequent exposure to a pathogen o r pest r e s u l t s i n a v i g o r o u s and r a p i d a c t i v a t i o n o f the p l a n t ' s endogenous g e n e t i c a l l y e n c o d e d d e f e n s e m e c h a n i s m s . However, a s i d e f r o m a r e l a t i v e l y b r i e f i n i t i a l s e n s i t i z a t i o n p e r i o d , d i s e a s e r e s i s t a n c e mechanisms remain l a t e n t u n t i l a f t e r i n f e c t i o n and a r e g e n e r a l l y l o c a l i z e d a t the s i t e s o f i n f e c t i o n . T h e r e i s no g r o s s d e r a n g e m e n t o f p l a n t c o n s t i t u t i v e m e t a b o l i s m . There i s l i t t l e consumption o f energy or m e t a b o l i t e s and t h e e x p e n d i t u r e o c c u r s when and where needed. A u t o t o x i c i t y from non s p e c i f i c or c o n s t i t u t i v e defense r e a c t i o n s i n the absence of i n f e c t i o n i s avoided. A " s e n s i t i z e d " p l a n t may be s a i d t o p o s s e s s t o some d e g r e e a s t a t e o f " i m m u n i t y " o r " a c q u i r e d / i n d u c e d resistance"· The remainder o f t h i s paper s h a l l p r e s e n t g e n e r a l p r i n c i p l e s , e x a m p l e s o f a p p l i c a t i o n s , and an e v a l u a t i o n o f c a p a b i l i t i e s and l i m i t a t i o n s o f s e n s i t i z a t i o n o f p l a n t s as a s t r a t e g y f o r d e f e n s e a g a i n s t i n f e c t i o u s d i s e a s e s and damage from o t h e r p e s t s .

Statement o f T h e s i s V i r t u a l l y a l l p l a n t s , even those considered " s u s c e p t i b l e " to particular diseases, possess genetic information f o r the biochemical pathways r e s p o n s i b l e f o r e f f e c t i v e d i s e a s e r e s i s t a n c e mechanisms. S u s c e p t i b i l i t y i s due t o a f a i l u r e o r d e l a y i n recognizing the pathogen, suppression of a c t i v a t i o n of host d e f e n s e s by t h e p a t h o g e n , inactivity o f gene p r o d u c t s , or m o d i f i c a t i o n o f c o m p o n e n t s o f t h e r e s i s t a n c e r e s p o n s e . The d a t a presented i n t h i s paper support the t h e s i s t h a t , given an a p p r o p r i a t e i n d u c i n g s t i m u l u s , s u s c e p t i b l e p l a n t s can be s e n s i t i z e d t o a c t i v a t e d e f e n s e m e c h a n i s m s a g a i n s t p a t h o g e n s so as t o become r e s i s t a n t to d i s e a s e . However, though enhancement o f e x p r e s s i o n o f g e n e t i c a l l y e n c o d e d p l a n t d i s e a s e r e s i s t a n c e m e c h a n i s m s may be a g e n e r a l l y e f f e c t i v e means o f i m m u n i z i n g p l a n t s , we do n o t c l a i m u n i v e r s a l efficacy. In some e n v i r o n m e n t s , gene p r o d u c t s may be i n e f f e c t u a l in restricting or c o n d i t i o n i n g the r e s t r i c t i o n o f pathogen d e v e l o p m e n t , e.g., a c t i v i t y o f gene p r o d u c t s f o r r e s i s t a n c e may have s p e c i f i c temperature or l i g h t requirements. In p l a n t s s e v e r e l y weakened by i n j u r y o r d i s e a s e , f a c u l t a t i v e p a r a s i t e s or n o r m a l l y i n c o m p a t i b l e pathogens may be a b l e to p a r a s i t i z e n o r m a l l y nonhost o r r e s i s t a n t p l a n t s due t o the p l a n t ' s i n a b i l i t y t o d e v e l o p an e f f e c t i v e m e t a b o l i c response. Some pathogens may a c t i v a t e host d e f e n s e mechanisms y e t d e v e l o p i n host t i s s u e s and cause d i s e a s e by inactivating c o m p o n e n t s o f t h e r e s i s t a n c e m e c h a n i s m s , e.g., metabolism of kievetone, a p h y t o a l e x i n o f b e a n s , by F u s a r i u m (26,27) and the d e t o x i f i c a t i o n o f p i s a t i n , a p h y t o a l e x i n o f pea, by N e c t r i a hematococca ( 2 8 ) .


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Methods of S e n s i t i z a t i o n Gene t i c . The d i f f e r e n c e s b e t w e e n many s u s c e p t i b l e and r e s i s t a n t c u l t i v a r s are the a b i l i t i e s o f the l a t t e r to a p p r o p r i a t e l y respond q u a n t i t a t i v e l y i n t i m e and i n s p a c e t o i n f e c t i o n , r a t h e r t h a n q u a l i t a t i v e d i f f e r e n c e s i n b i o c h e m i c a l pathways. T h i s type of g e n e t i c r e s i s t a n c e may be c o n s i d e r e d a f o r m o f an e n d o g e n o u s c o n s t i t u t i v e ly sensitized state. Genetic s e n s i t i z a t i o n of a c u l t i v a r r e q u i r e s b o t h a s o u r c e o f a p p r o p r i a t e g e r m p l a s m and a m e t h o d o f e f f e c t i v e t r a n s f e r and i n c o r p o r a t i o n o f t h e g e n e t i c m a t e r i a l i n t o the r e c i p i e n t c u l t i v a r . Both r e q u i r e m e n t s cannot a l w a y s be met. F u r t h e r m o r e , u n d e s i r a b l e t r a i t s o f t e n accompany desired resistance traits. This v e r y important area of p l a n t s e n s i t i z a t i o n , b r o a d l y c o n s t r u e d , l i e s beyond the scope of t h i s paper. Physical. A n e c d o t a l a c c o u n t s abound of enhanced p l a n t r e s i s t a n c e t o d i s e a s e a c h i e v e d by t r a n s i e n t e x p o s u r e t o a w i d e v a r i e t y o f physical stimuli, e.g., heat, light, microwaves, other electromagnetic r a d i a t i o n , e l e c t r i c c u r r e n t , s o u n d w a v e s , and vibration. In our own l a b o r a t o r y , we have made cucumbers r e s i s t a n t t o a n t h r a c n o s e by v i b r a t i o n ( S t r o m b e r g and Kuc, unpublished). H o w e v e r , t h e s e phenomena a r e p o o r l y u n d e r s t o o d and may include enhanced r e s i s t a n c e r e s u l t i n g f r o m n o n - s p e c i f i c a l t e r e d ( s t r e s s ) p h y s i o l o g y , n o n s p e c i f i c p h y t o a l e x i n e l i c i t a t i o n , m o d i f i c a t i o n of t h e a c t i o n o f gene p r o d u c t s , o r s e n s i t i z a t i o n . T h i s i n t e r e s t i n g b u t l i t t l e e x p l o r e d a r e a w i l l n o t be f u r t h e r d i s c u s s e d i n t h i s paper. Biotic. Enhanced r e s i s t a n c e to d i s e a s e which r e s u l t s from p r i o r exposure to a v i r u l e n t pathogens o r n o n p a t h o g e n i c o r g a n i s m s o r t o v i r u l e n t pathogens under c o n d i t i o n s u n f a v o r a b l e f o r disease d e v e l o p m e n t , i s the major focus of t h i s paper and w i l l be d i s c u s s e d at l e n g t h . C h e m i c a 1 . T h e r e a r e a number o f r e p o r t s o f e x o g e n o u s c h e m i c a l s , b o t h s y n t h e t i c and n a t u r a l , which enhance endogenous p l a n t d e f e n s e mechanisms. We s h a l l r e v i e w some o f these r e p o r t s a t a l a t e r p o i n t i n t h i s paper. In some c a s e s , n e i t h e r t h e compounds n o r their m e t a b o l i c p r o d u c t s are d i r e c t l y t o x i c to the pathogens under study. In o t h e r cases, enhanced p l a n t d e f e n s e r e s p o n s e s appear c o e x i s t e n t with d i r e c t p e s t i c i d a l activity of the compounds o r their metabolites. Few o f these c a s e s have been t h o r o u g h l y examined at the b i o c h e m i c a l l e v e l . However, the p o s s i b i l i t y t h a t s e n s i t i z a t i o n of p l a n t s towards pathogens may be a c h i e v e d by exogenous c h e m i c a l s has g r e a t p o t e n t i a l f o r p r a c t i c a l a p p l i c a t i o n s . Of c o u r s e , a l l difficulties inherent i n r e l e a s i n g a l i e n c h e m i c a l s i n t o the e n v i r o n m e n t may be e n c o u n t e r e d i n t h i s a p p r o a c h . R e g a r d l e s s of the n a t u r e o f the o r i g i n a l s e n s i t i z i n g s t i m u l u s , however, induced r e s i s t a n c e i s mediated and e x p r e s s e d through the endogenous g e n e t i c a l l y encoded b i o c h e m i c a l mechanisms of the p l a n t .


52



G e n e r a l Occurrence

o f R e s i s t a n c e Induced

by B i o t i c

Agents

Numerous r e p o r t s o f " a c q u i r e d r e s i s t a n c e " o r " i n d u c e d r e s i s t a n c e " o r " i m m u n i z a t i o n " o f p l a n t s , e s p e c i a l l y t o d i s e a s e s c a u s e d by v i r u s e s , date back as f a r as a c e n t u r y (_1). Only a s m a l l sampling of e x a m p l e s i s p r e s e n t e d i n T a b l e I to i l l u s t r a t e the v a r i e t y o f host-disease combinations f o r w h i c h t h i s phenomenon has b e e n reported. Many examples have not been i n d e p e n d e n t l y v e r i f i e d or extensively investigated. D i r e c t a n t i b i o s i s or changes i n host p h y s i o l o g y or pathogen v i r u l e n c e may be c o n f u s e d w i t h , or obscure, s e n s i t i z a t i o n towards subsequent r e i n f e c t i o n by the same pathogen. However, enough c a s e s h a v e been c l o s e l y examined to s u g g e s t the g e n e r a l i t y of b i o t i c a l l y induced r e s i s t a n c e i n p l a n t s (2-11). S i m i l a r phenomena h a v e a l s o b e e n r e p o r t e d f o r n e m a t o d e s (29) and f o r i n s e c t s (30). N o n - s p e c i f i c i t y of

Protection

One o f t h e most f a s c i n a t i n g and y e t b e w i l d e r i n g a s p e c t s o f r e s i s t a n c e i n d u c e d i n p l a n t s by b i o t i c a g e n t s i s t h e f r e q u e n t l y o b s e r v e d a b i l i t y of i n f e c t i o n by one organism to induce r e s i s t a n c e a g a i n s t d i s e a s e caused by o t h e r organisms d i s t a n t l y r e l a t e d both to the i n d u c i n g organism and to one another. Limited foliar infection o f cucumber, watermelon, and muskmelon by C o l l e t o t r i c h u m l a g e n a r i u m , a pathogen o f l e a v e s and f r u i t s , o r by t h e n o n - p a t h o g e n t o b a c c o n e c r o s i s v i r u s (TNV), s y s t e m i c a l l y p r o t e c t e d a g a i n s t d i s e a s e c a u s e d by C. l a g e n a r i u m , Cladosporium cucumerinum, Mycosphaerelia melonis, Fusarium o x y s p o r u m f . s p . c u e urne r i n u m , P s e u d o p e r o n o s p o r a c u b e n s i s, Pseudomonas lachrymans, E r w i n i a t r a c h e i p h i l a , TNV ( l o c a l n e c r o s i s ) , or P h y t o p h t h o r a i n f e s t a n s (9,49-55). Thus p r o t e c t i o n was a c h i e v e d a g a i n s t n e c r o t r o p h i c and b i o t r o p h i c pathogens; f u n g i , v i r u s e s , and b a c t e r i a ; f o l i a r , stem, f r u i t , and r o o t p a t h o g e n s ; and a g a i n s t v a r i o u s d i s e a s e types - s y s t e m i c and l o c a l ; w i l t s , r o t s , b l i g h t s , l e a f s p o t s , mildews, scabs, and l o c a l n e c r o s i s . The o n l y d i s e a s e a g a i n s t w h i c h we were u n a b l e t o i n d u c e r e s i s t a n c e w i t h TNV o r C. l a g e n a r i u m was p o w d e r y m i l d e w c a u s e d by S p h a e r o t h e c a fulginea. However, B a s h a n and Cohen (56) h a v e more r e c e n t l y r e p o r t e d i n d u c t i o n of r e s i s t a n c e a g a i n s t powdery m i l d e w o f cucumbers by TNV. G r e e n b e a n s ( P h a s e o l u s v u l g a r i s L.) c a n be systemically p r o t e c t e d a g a i n s t c u 1 t i v a r - p a t h o g e n i c r a c e s of the a n t h r a c n o s e fungus, C o l l e t o t r i c h u m 1indemuthianum, by p r i o r i n o c u l a t i o n e i t h e r w i t h c u l t i v a r - i n c o m p a t i b l e r a c e s o f the same fungus (57-59) or by c u l t i v a r - p a t h o g e n i c r a c e s i f the i n f e c t i o n was a t t e n u a t e d i n s i t u by h e a t t r e a t m e n t p r i o r t o a p p e a r a n c e o f symptoms ( 6 0 , 6 1 ) . Bean c u l t i v a r s s u s c e p t i b l e to a l l known r a c e s o f C^ 1indemuthianum and a l s o t h o s e p o s s e s s i n g " g e n e t i c " r e s i s t a n c e t o some f u n g a l r a c e s were r e n d e r e d r e s i s t a n t t o a l l r a c e s o f t h e p a t h o g e n by p r i o r i n f e c t i o n w i t h the pathogen o f c u c u r b i t s , Co 1 l e t o t r i c h u m l a g e n a r i u m ( 57_, 62 ). T o b a c c o was r e c i p r o c a l l y p r o t e c t e d a g a i n s t TMV, TNV, or T h i e l a v i o p s i s b a s i c o l a (34) ; a g a i n s t P h y t o p h t h o r a p a r a s i t i c a v a r . nicot ianae with TNV ( 63); and a g a i n s t TMV and Erys iphe c i c h o r a c e a r u m (powdery mildew) w i t h P e r o n o s p o r a t a b a c i n a ( 6 4 - 6 6 ) .


4.

SALT AND KUC Table

I.

Some Reports of B i o t i c a l l y

Plant

Disease

b l u e mold tobacco mosaic tobacco r i n g s p o t b l a c k shank black root r o t

Tobacco



Cabbage, carnation, tomato, watermelon, flax, s i l k tree, cotton

wilt

53

Induced

R e s i s t a n c e In P l a n t s

Pathogen

Reference(s)

Peronospora t a b a c i n a virus virus Phytophthora p a r a s i t i c a Thielaviopsis basicola Fusarium

5,31,32 1,3 1,3 33 34

spp.

35

Sugar cane

mosaic corn streak

Coffee

tree

rust

Hemileia

vastatrix

36,37

Euphorbia cyparissias

rust

Uromyces

pisi

38

Daisies

gall

Agrobacterium

Cedars, apples

rust

Gymnosporangium macropus

40

Carrots

root rot

Botrytis

41

Cucurbits (cucumbers, melons)

anthracnose mosaic a n g u l a r l e a f spot

C o l l e t o t r i c h u m lagenarium virus Pseudomonas lachrymans

Peaches, plums

canker

Cytospora

Green beans

anthracnose

Colletotrichum 1indemuthianum

45

A p p l e s , pears

fire

Erwinia

46-48

blight

virus virus

1 1

tumefasciens

cinerea

cincta

amylovora


39

42,43,54 1 10 44


54


However, i n o c u l a t i o n w i t h c i c h o r a c e a r u m , TMV, cucumber mosaic virus, A l t e r n a r i a so 1 a n i , Helminthosporium turc icum,or P s e u d o p e r o n o s p o r a c u b e n s i s f a i l e d to p r o t e c t t o b a c c o a g a i n s t Peronospora t a b a c i n a or c i c h o r a c e a r u m (65)» Thus, w h i l e b i o t i c agents often protect against c l o s e l y or d i s t a n t l y r e l a t e d organisms, the e f f e c t i v e c o m b i n a t i o n s o f p l a n t , b i o t i c i n d u c e r , and c h a l l e n g e pathogen e x h i b i t some s p e c i f i c i t y . It i s p u z z l i n g that r e c i p r o c a l c o m b i n a t i o n s may be i n e f f e c t i v e , and t h a t some pathogens seem i n e f f e c t i v e i n e l i c i t i n g r e s i s t a n c e a g a i n s t t h e m s e l v e s o r c l o s e l y r e l a t e d organisms, but h i g h l y e f f e c t i v e a g a i n s t d i s t a n t l y r e l a t e d pathogens! Some more u n u s u a l e f f e c t i v e i n d u c e r / p a t h o g e n c o m b i n a t i o n s i n c l u d e t h e use o f t h e n e m a t o d e s P r a t y l e n c h u s p e n e t r a n s o r P. b r a c h y u r r u s a g a i n s t the b l a c k shank fungus P. p a r a s i t i c a i n t o b a c c o (33,67,68), and use of TMV a g a i n s t the a p h i d Myzus p e r s i c a e as w e l l as a g a i n s t TMV, j \ _ p a r a s i t i c a v a r . n i c o t i a n a e , Pseudomonas t a b a c i ( w i l d f i r e b a c t e r i u m ) , and Peronospora t a b a c i n a (69). Dynamics of Induced

Resistance

Initial reaction. In a l l known c a s e s of e f f e c t i v e b i o t i c s e n s i t i z a t i o n of p l a n t s r e p o r t e d to date, a c r i t i c a l f a c t o r appears to be the n e c r o s i s o f h o s t c e l l s i n the zone of i n i t i a l i n f e c t i o n . However, w h i l e n o n - n e c r o t i c i n f e c t i o n s a r e i n e f f e c t i v e i n d u c e r s , n e c r o s i s per se i s not e f f e c t i v e i n i n d u c i n g r e s i s t a n c e . I n j u r y by a b i o t i c a g e n t s s u c h as h e a t , c h e m i c a l s , d r y i c e , o r various e x t r a c t s f r o m p l a n t s and m i c r o b e s d o e s n o t p r o t e c t c u c u m b e r s against l a g e n a r i u m (8-10). I n f e c t i o n o f t o b a c c o by a w i d e v a r i e t y of P e r o n o s p o r a l e s f u n g i o t h e r than P^_ t a b a c i n a frequently c a u s e s s e v e r e n e c r o s i s , but does not i n d u c e s y s t e m i c r e s i s t a n c e a g a i n s t b l u e mold (Tuziin and Kuc, unpublished). The e f f e c t i v e n e s s o f immunization i s d i r e c t l y r e l a t e d to the e x t e n t of i n f e c t i o n by, or i n o c u l u m c o n c e n t r a t i o n o f , the inducer o r g a n i s m up t o a p o i n t o f m a x i m a l r e s p o n s e o r saturation (49,53, 70). H o w e v e r , one l e s i o n c a u s e d by C. l a g e n a r i u m o r e i g h t l e s i o n s caused by TNV on one i n d u c e r l e a f can s i g n i f i c a n t l y induce s y s t e m i c d i s e a s e r e s i s t a n c e i n cucumbers. S p a t i a l and Temporal r e l a t i o n s h i p s . A common c h a r a c t e r i s t i c o f b i o t i c s e n s i t i z a t i o n of p l a n t s i s a l a t e n t or l a g p e r i o d between i n i t i a t i o n o f the i n d u c e r i n f e c t i o n and m a n i f e s t a t i o n o f d i s e a s e resistance. In the cucumber/TNV or l a g e n a r i u m system, enhanced r e s i s t a n c e a g a i n s t pathogens i s f i r s t m a n i f e s t about 48-72 hr a f t e r i n i t i a l i n f e c t i o n and maximal r e s i s t a n c e i s a c h i e v e d by 120-144 hr (49,53,70). In tobacco, r e s i s t a n c e a g a i n s t Ρ^_ t a b a c i n a a c h i e v e d by l i m i t e d stem i n f e c t i o n by the same organism i s not e v i d e n t b e f o r e 9 d a y s and i n c r e a s e s u n t i l a b o u t 21 d a y s a f t e r i n d u c t i o n ( 5 ) . An induction period o f 3-6 days i s r e q u i r e d for expression of r e s i s t a n c e t o TMV o r TNV i n d u c e d i n t o b a c c o by T h i e l a v i o p s i s b a s i c o l a ( 34). R e m o v a l o f the i n d u c e r l e a f 72-96 h r a f t e r i n i t i a t i o n o f the i n d u c i n g i n f e c t i o n i n cucumbers d i d not r e s u l t i n the l o s s o f s y s t e m i c r e s i s t a n c e i n the r e s t o f the p l a n t (53). Likewise, l e a v e s d i s t a l from the i n d u c e r l e a f r e t a i n e d r e s i s t a n c e a f t e r detachment from the p l a n t once immunization was e s t a b l i s h e d .



4.

SALT AND

KUC


55

D u r a b i l i t y o f b i o t i c a l l y i n d u c e d r e s i s t a n c e v a r i e s w i t h the ρlant/inducer/pathogen combination. In c u c u m b e r s , r e s i s t a n c e i n d u c e d by l a g e n a r i u m o r TNV gradually declines unless a " b o o s t e r " i n o c u l a t i o n i s g i v e n 3-6 weeks a f t e r i n d u c t i o n ( 7 0 ) . With a b o o s t e r i n o c u l a t i o n , i m m u n i z a t i o n of cucumbers appears s u f f i c i e n t l y d u r a b l e to p r o t e c t the p l a n t t h r o u g h f l o w e r i n g and fruiting. However, cucumbers cannot be immunized a f t e r i n i t i a t i o n of f l o w e r i n g (7_1_)· T h i s s u g g e s t s a h o r m o n a l e f f e c t upon immunization. In t o b a c c o , i m m u n i z a t i o n a g a i n s t t a b a c i n a by v i r u l e n t P. t a b a c i n a i s hazardous b e f o r e the p l a n t s are at l e a s t 20 cm t a l l due t o f r e q u e n t s y s t e m i c s p r e a d o f t h e f u n g u s i n y o u n g p l a n t s , but a s i n g l e i n o c u l a t i o n o f p l a n t s o v e r 20 cm i n h e i g h t i s e f f e c t i v e t h r o u g h o u t the p l a n t ' s l i f e (5,72). Protection of t o b a c c o a g a i n s t P h y t o p h t h o r a p a r a s i t i c a v a r . n i c o t i a n a e by TMV seems to r e q u i r e r e p e a t e d a d m i n i s t r a t i o n o f the v i r u s (69). W h i l e b i o t i c s e n s i t i z a t i o n of p l a n t s a g a i n s t pathogens may a c t s y s t e m i c a l l y , not a l l organs or t i s s u e s are n e c e s s a r i l y p r o t e c t e d equally. Recent work i n our l a b o r a t o r y has shown t h a t the e x t e n t of r e s i s t a n c e a g a i n s t a n t h r a c n o s e or a n g u l a r l e a f spot of l e a v e s i n d i f f e r e n t p o s i t i o n s on i m m u n i z e d c u c u m b e r p l a n t s v a r i e s i n a complex manner not n e c e s s a r i l y d i r e c t l y p r o p o r t i o n a l to p r o x i m i t y o f the i n d u c e r l e s i o n s (73). Removal of e p i d e r m a l l a y e r s from immunized cucumber l e a v e s reduces r e s i s t a n c e to a n t h r a c n o s e (74). Mechanisms o f R e s i s t a n c e . The broad s p e c t r u m o f e f f e c t i v e n e s s o f induced s y s t e m i c r e s i s t a n c e a g a i n s t b a c t e r i a , v i r u s e s , f u n g i , and n e m a t o d e s , makes i t seem u n l i k e l y t h a t o n l y a s i n g l e r e s i s t a n c e mechanism i s a c t i v a t e d . Phytoalexins. Many, but not a l l , p l a n t f a m i l i e s are r e p o r t e d to u t i l i z e p h y t o a l e x i n a c c u m u l a t i o n as a means of d e f e n s e a g a i n s t pathogens. A major p h y t o a l e x i n i n green beans, P h a s e o l u s v u l g a r i s , i s t h e i s o f l a v o n o i d p h a s e o l l i n (18). R e a c t i o n s of " g e n e t i c a l l y r e s i s t a n t " bean v a r i e t i e s a g a i n s t incompatible r a c e s of C. 1 indemuthianum or of immunized " s u s c e p t i b l e " b e a n p l a n t s against v i r u l e n t r a c e s are marked by r a p i d a c c u m u l a t i o n of h i g h l e v e l s o f p h a s e o l l i n i n t i s s u e s a d j a c e n t to the i n v a d i n g fungus, r e s u l t i n g i n c o n t a i n m e n t o f the f u n g a l h y p h a e ( 62,75 ; F i g u r e 1). On t h e o t h e r hand, w h i l e u l t i m a t e p h y t o a l e x i n a c c u m u l a t i o n on a t o t a l p l a n t t i s s u e b a s i s c a n be g r e a t e r i n nonimmunized i n f e c t e d s u s c e p t i b l e p l a n t s , t h e o n s e t o f a c c u m u l a t i o n i s d e l a y e d ( F i g u r e 1); l o c a l concentrations a r e i n s u f f i c i e n t t o c o n t a i n t h e f u n g u s , and the p l a n t i s s u c c e s s f u l l y c o l o n i z e d by the pathogen. S i g n i f i c a n t l y , as shown i n F i g u r e 1, p h y t o a l e x i n s do not a c c u m u l a t e i n the immunized " s u s c e p t i b l e " bean p l a n t d i s t a n t from the s i t e of i n d u c t i o n p r i o r to exposure to the pathogen. That i s , s e n s i t i z e d r e s i s t a n c e i s not due to h i g h constitutive l e v e l s of p h y t o a l e x i n s . Induced r e s i s t a n c e through p r i o r b i o t i c i n f e c t i o n , t h e r e f o r e , appears to be at l e a s t a two-step phenomenon: s e n s i t i z a t i o n f o l l o w e d by a r a p i d e x p r e s s i o n of r e s i s t a n c e mechanisms a f t e r subsequent i n f e c t i o n . I n h i b i t i o n o f p e n e t r a t i o n and s p r e a d w i t h i n t i s s u e s . In c u c u r b i t s , c l a s s i c a l p h y t o a l e x i n s h a v e n o t b e e n i d e n t i f i e d and d i s e a s e r e s i s t a n c e appears due to o t h e r mechanisms. Immunization


BIOREGULATORS FOR PEST C O N T R O L


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Bioregulators for Pest Control - American Chemical Society

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